A demand for vehicles with lower fuel consumption is now growing with the global movement toward the reduction in carbon dioxide emissions, resulting from the growing interest in environmental issues. To respond to such a demand, tires are required to have low rolling resistance. The low rolling resistance of tires may be achieved, for example, by using a rubber composition with low heat generation property. The low heat generation property is conventionally achieved by, for example, using a polymer having increased affinity for carbon black and silica. Refer, for example, to WO 2007/129670 (PTL 1), in which the affinity between fillers and rubber component is increased to reduce the heat generation from the rubber composition. Such a rubber composition can form a tire with low hysteresis loss. However, as the demand for vehicles with lower fuel consumption grows even further, tires are also required to be improved further to have even lower heat generation property. Additionally, the heat generation may be also reduced by, for example, lengthening the kneading time, which however reduces the heat generation only in a limited way.
A catalyst composition containing a rare earth metal is one conventional polymerization catalyst composition known in the art used for synthesizing a polymer. One example of such a catalyst composition contains (i) a compound that contains a rare earth element (REE-containing compound), (ii) an organic aluminum compound, and (iii) an ionic compound. The REE-containing compound contains a rare earth element compound (REE compound) or a reactant of the REE compound with a Lewis base. The ionic compound, numbered as (iii), of the catalyst composition involves cost problem, so there has been a need for a compound substituting the number (iii) compound. Silica has been commonly used to substitute the ionic compound. Refer to Macromolecules (2005) 38, pp. 3060-3067 by T. J. Woodman et al. (NPL 1). However, the method described in NPL 1 requires calcination of silica, which also involves cost problem.